Advanced technical development has been made with times in the technical field of optical information. The amount of information to be recorded in various optical storage media such as a compact disk (CD), a digital versatile disk (DVD), and a Blu-ray disk has been increasing. A Blu-ray disk having two information recording layers for recording information, with a disk diameter of 12 cm, has a storage capacity as large as 50 gigabytes (GB).
In recent years, development of volume holography has been active to realize optical recording of a larger capacity than the Blu-ray disk. Holographic recording is conducted by superimposing information light and reference light inside an optical storage medium and by writing an interference pattern generated by the superimposing into the optical storage medium. The information recorded in the optical storage medium is reproduced by allowing the reference light to be incident onto the optical storage medium. When the reference light is incident onto the optical storage medium, the interference pattern recorded in the optical storage medium generates diffraction light having information carried by the information light at the time of recording. Volume holography is one of the holographic recording of recording information in a thickness direction of the optical storage medium as well as in planar directions thereof, namely, recording information three-dimensionally. The multiple recording provides an increased recording capacity.
Japanese Unexamined Patent Publication No. 2004-265472 discloses an example of an optical information apparatus for performing the holographic recording. FIG. 7 is a diagram showing a relation between an optical storage medium, and a beam emitted from the optical information apparatus disclosed in the publication according to the prior art.
In the publication, after a red beam 7 emitted from a laser source for servo control is reflected on a mirror 13, the reflected red beam is transmitted through an objective lens 12. The red beam 7 through the objective lens 12 is converted into a convergent beam, and the convergent beam is incident onto a light incoming/outgoing plane “A” of an optical information recording medium 101. Then, the red beam 7 incident onto the optical information recording medium 101 is transmitted through a base member 5, a hologram recording layer 4, and a red transmission filter layer 6, and is focused on a reflection layer 2. The reflection layer 2 is formed on a base member 1 with pits capable of detecting a servo signal. The red beam 7 reflected on the reflection layer 2 is transmitted through the red transmission filter layer 6, the hologram recording layer 4, and the base member 5 again, and goes out of the incoming/outgoing plane “A”. The outgoing red beam is, then, transmitted through the objective lens 12, and is reflected on the mirror 13 by 100%. Thus, servo information is detected by an unillustrated servo information detector. The detected servo information is used for focus servo control, tracking servo control, slide servo control, and the like. The hologram material constituting the hologram recording layer 4 is a material free of photosensitivity to red light so that the incidence of the red beam 7 for servo control does not influence the hologram recording layer 4.
Likewise, information light and reference light for recording that have been generated from a green or a blue beam 8 emitted from a laser source for recording/reproduction are transmitted through the mirror 13. Then, the information light and the reference light through the objective lens 12 are irradiated onto the optical information recording medium 101 so that an interference pattern is generated inside the hologram recording layer 4. Information is recorded as the interference pattern in the hologram recording layer 4.
In reproducing the information recorded in the optical storage medium 101, the reference light is irradiated onto the optical storage medium 101 through the objective lens 12. The irradiation of the reference light onto the optical storage medium 101 generates diffraction light containing the information carried by the information light at the time of information recording from the hologram recording layer 4. Allowing the diffraction light to be detected by an image sensor 14 reproduces the information recorded in the optical storage medium 101.
As disclosed in the publication, however, in the case where the beam for servo control, and the beam for information recording/reproduction are different, there is no guarantee that a relative positional relation between the beam for servo control and the beam for information recording/reproduction is kept constant concerning focusing control and tracking control. Therefore, there is room for technical improvement in securing compatibility among different optical information apparatuses, or securing reliability including a measure against aging deterioration even in identical optical information apparatuses.